Since I can't create threads yet, there is a video interview by Ukrainian tech website itcchannel with 4A's Andrew Prokhorov the creative director for Metro: Last Light basically confirming the possible release of an SDK, 5 separate DLCs with different characters & stories from the main game and the release of the multiplayer version as a standalone down the line.

Here is the link from the Metro:LL forums where one of the members translated and highlighted the most important news from the interview. He also states that a PS4 version is confirmed: